In the communications industry, various protocols, network structures and signaling techniques have evolved to address the ever growing needs of society. The increased mobility of the typical consumer has raised a number of important technical issues. For example, in mobile communication environments, synchronization between the receiver and the transmitter is an issue of particular concern. Specifically, most communication environments require that the receiver be “locked” or “in sync” with the transmitter in terms of frequency and timing. This requirement presents unique difficulties in the mobile setting for a number of reasons.
For example, when the receiver is a hand-held terminal such as in various commercially available user terminals, synchronization can be affected by variations in environmental conditions as well as channel conditions. Environmental conditions include receiver temperature, while channel conditions include line of sight (LOS) positioning with respect to the transmitter (such as a satellite), and relative motion between the receiver and the transmitter (i.e., the doppler effect). It is therefore desirable to provide a mechanism within the receiver that enables changes in timing and frequency to be tracked, even during conditions that typically cause signal degradation and loss of synchronization.
The conventional receiver includes a modem (operating under one or more protocols such as CDMA, TDMA, and QPSK), and a synchronization system. The synchronization system includes an error signal estimator, a filter loop structure and a piece of hardware on which the filtered version of the error estimates are applied. The demodulator typically provides the synchronization system with an error signal corresponding to a synchronization parameter such as frequency or time. In turn, the synchronization system generates a final frequency or time adjustment signal, which is used by the hardware. The hardware applies this correction and hence attempts to maintain synchronization with the transmitter. The hardware typically does not have infinite precision, in that it can only handle discrete steps of corrections. Hence the hardware can be looked as to have an in-built quantizer. The error signal estimator that is a part of the synchronization system estimates time and frequency error parameters on a burst by burst basis. These estimates are often relatively noisy since the incoming bursts are often corrupted with channel noise. Thus, the error signal needs to be filtered before being applied to the hardware. Typically, for frequency control the hardware is a Voltage Controlled Oscillator (VCO) which is used to change the receive and transmit frequency of the receiver. For time control the filtered error signal is applied to the sampling time instant of the Analog to Digital (A/D) converter.
While the filtering loops commonly used to implement the above-described smoothing function are acceptable under certain circumstances, considerable room for improvement remains. For example, it has been determined that conventional first order filters cannot compensate for the ramp nature of drift. Thus, the output of most filter loops lags the input. Furthermore, when signal degradation occurs as a result of the above-described environmental and channel conditions at the same time that there is drift in the error signal, the modem can loose synchronization with the network. This loss in synchronization occurs because the modem has no mechanism of predicting the manner in which the input frequency and timing is varying. Loss of synchronization results in dropped calls (during periods of activity) and failures to recognize the network (during periods of inactivity). It is therefore desirable to provide a mechanism for estimating a rate of change in an error signal in order to maintain synchronization between a receiver and a transmitter.